1. Field of the Invention
This invention generally relates to a device for optically recording and/or reproducing information, and, in particular, to an monolithic optical pick-up for optically recording or reproducing information on or from a recording medium, such as an optical disk.
2. Description of the Prior Art
A typical prior art optical pick-up for use in an optical information recording/reproducing apparatus is schematically shown in FIG. 28. As shown, a divergent light flux emitted from a semiconductor laser 1 is, for example, separated into three beams by a diffraction grating 2. These light fluxes, after passing through a polarizing beam splitter 3, pass through a 1/4 wavelength plate 4 to become circularly polarized and then focused onto the information recording surface of an optical disk 6 as a recording medium by means of an objective lens 5. Then, the light flux reflecting from the information recording surface of the optical disk 6, after passing through the objective lens 5, passes through the 1/4 wavelength plate 4 to become linearly polarized and is again introduced into the polarizing beam splitter 3. The polarizing beam splitter 3 causes the reflecting light from the information recording surface of the optical disk 6 to be reflected sideways to thereby separate it from the incident light, and the light thus reflected by the beam splitter 3 passes through a cylindrical lens 7 to be lead into a photodetector 8. A detection signal output from the photodetector 8 is processed not only as an information read-out signal, but also as an auto-focusing signal for keeping light from a light source to be always focused on the information recording surface of the optical disk and as a tracking signal for adjusting the position of the optical pick-up such that the convergent light toward the information recording surface of the optical disk is always focused onto an information track on the information recording surface.
However, in such a prior art optical pick-up, since use is made of a cubic polarizing beam splitter 3, which is a relatively bulky optical component, the device as a whole tends to be large in size and in weight. For this reason, there are such disadvantages as long access time, high manufacturing cost and insufficient mechanical stability.
In order to obviate such disadvantages, for example, Japanese Patent Laid-open Pub. No. 61-92439 discloses an idea to provide a light source, a grating type collimating element and a grating type light condensing element for condensing light onto a recording medium, a grating beam splitter element for separating the reflecting light flux from the recording medium, and a light flux separating and condensing element for condensing the separated light to a photodetector on the same substrate in a monolithic structure; on the other hand, Japanese Patent Laid-open Pub. No. 61-178740 teaches to form a beam splitter for separating the reflecting light from the recording medium from the incident light from a diffraction grating arranged perpendicular to the optical axis However, in the former case, since a semiconductor laser serving as the light source is directly coupled to an optical waveguide, the coupling efficiency to the optical waveguide cannot be enhanced (approximately 3%), so that the use rate of light is significantly poor. For this reason, light having sufficient power cannot reach the information recording surface, and, thus, recording and/or reproducing operations with high S/N ratio cannot be carried out. In particular, in information recording operations, since large recording power is required, this cannot be adopted virtually. In addition, when the oscillating wavelength of a beam from a semiconductor laser as a light source fluctuates by the so-called mode hopping phenomenon which results from the temperature dependent characteristic, it is affected directly to cause the convergent light toward the information recording surface to be shifted away from an information track, thereby creating a problem of incapability to carry out recording and/or reproducing information accurately.
Also in the latter case, since the reflecting light from the information recording surface is diffracted toward a photodetector by a diffraction grating, when the oscillating wavelength of a beam from a semiconductor laser as a light source fluctuates due to the mode hopping phenomenon, the diffraction angle also fluctuates along therewith, so that fluctuations are produced in the position of the convergent light on the photodetector, thereby making it difficult to carry out recording and/or reproducing of information stably.